Generation of angiostatin by reduction and proteolysis of plasmin. Catalysis by a plasmin reductase secreted by cultured cells

Radiation-induced tumour necrosis factor-alpha expression: clinical application of transcriptional and physical targeting of gene therapy

Promising data are emerging on a new anticancer agent, Ad.EGR-TNF, an adenoviral vector, which contains radio-inducible DNA sequences from the early growth response (EGR1) gene promoter and cDNA for the gene encoding human tumour necrosis factor-alpha. Ad.EGR-TNF combines the well-documented broad-spectrum anticancer activity of TNFalpha with the proven clinical usefulness of radiotherapy. Systemic delivery of the TNFalpha protein has had limited success clinically because of severe dose-limiting toxic effects. This limitation has been overcome by the use of a gene delivery approach, combined with a radiation-inducible promoter to express the TNFalpha protein in the irradiated tumour tissue. Preclinical and early phase I clinical testing indicates that effective concentrations of TNFalpha can be delivered to the tumour site without significant systemic exposure or toxic effects. The combination of radiation and TNFalpha gene delivery has produced striking antitumour effects in model systems in animals. In the clinical setting, potent anticancer activity has been observed with a high rate of complete and partial objective tumour responses. A novel mechanism of destruction of the tumour vasculature seems to be central to this distinct antitumour activity. This review summarises the rationale, mechanistic basis, preclinical data, and preliminary clinical findings for this new treatment model.

Angiogenesis as a target for cancer therapy

Antiangiogenic drugs are unique for having highly specific targets while carrying the potential to be effective against a wide variety of tumors. Moreover, some of the major limitations of cytotoxic therapies likely will be avoided by this entirely new class of anticancer weapons. After the realization of the potential advantages of antiangiogenic therapy, the field of angiogenesis research is growing exponentially. Still, there is much to learn about the machinery that tumors use to recruit new blood vessels, and the results of the clinical trials will show the best way to apply that knowledge for cancer therapy.

Phosphorylation of pyrimidine L-deoxynucleoside analog diphosphates. Kinetics of phosphorylation and dephosphorylation of nucleoside analog diphosphates and triphosphates by 3-phosphoglycerate kinase

Anticancer and antiviral D- and L-nucleoside analogs are phosphorylated stepwise in the cells to the pharmacologically active triphosphate metabolites. We recently reported that in the last step, L-deoxynucleoside analog diphosphates are phosphorylated by 3-phosphoglycerate kinase (PGK). To explain the preference of PGK for L- over D-deoxynucleoside analog diphosphates, the kinetics of their phosphorylation were compared with the dephosphorylation of the respective triphosphates using recombinant human PGK. The results attributed favorable phosphorylation of L-deoxynucleoside analog diphosphates by PGK to differences in k(cat), which were consequences of varied orientations of the sugar and diphosphates in the catalytic site of PGK. The amino acids involved in the catalytic reaction of PGK (including Glu(344), Lys(220), and Asn(337)) were therefore mutated. The impact of mutations on the phosphorylation of L- and D-deoxynucleoside analog diphosphates was different from those on dephosphorylation of the respective triphosphates. This suggested that the interactions of the nucleoside analogs with amino acids during the transition state are different in the phosphorylation and dephosphorylation reactions. Thus, reversible action of the enzyme may not involve the same configuration of the active site. Furthermore, the amino acid determinants of the action of PGK for L-deoxynucleotides were not the same as for the D-deoxynucleotides. This study also suggests the potential impact of nucleoside analog diphosphates and triphosphates on the multiple cellular functions of PGK, which may contribute to the action of the analogs.

Differential production of angiostatin by concomitant antitumoral resistance-inducing cancer cells

The phenomenon by which tumor-bearing hosts are capable of inhibiting secondary tumor implants or metastases, known as concomitant antitumoral resistance (CAR), is presumably due to antiangiogenesis at places distant from the primary tumor. Although angiostatin, a potent inhibitor of angiogenesis, has been reported to be one of the factors responsible for suppressing the growth of secondary tumors in mice bearing previous tumors, it has not been definitively proven yet. With the aim of investigating whether CAR-inducing cancer cells display a differential angiostatin production and to support the role ascribed to that molecule concerning the inhibition of secondary tumor implants, 5 tumor models with different CAR-inducing capacities were studied herein. One of the 2 human lung cancer cell lines analyzed revealed a strong CAR against secondary s.c. tumor implants in nude mice, and 2 of 3 of the murine mammary tumors used exhibited inhibitory effect on secondary s.c. and i.v. tumor inoculations in syngeneic hosts. Since angiostatin is a proteolytic fragment from plasminogen, we examined by Western blot the ability of all conditioned media collected from the tumor cells studied to convert plasminogen to angiostatin. An association between in vivo generation of CAR and in vitro conversion of plasminogen into angiostatin was found. Since different enzymatic mechanisms were described to explain the generation of angiostatin, we also studied gelatinase and urokinase-type plasminogen activator secretion in conditioned media by zymography. The conversion of plasminogen into angiostatin by conditioned media was mainly inhibited by broad-spectrum serine proteinase inhibitors, suggesting a possible role for 1 or more enzymes of that group in the process. These findings suggest the existence of a differential angiostatin generation by CAR-inducing cancer cells, providing additional support to previous data obtained by other authors.

Angiostatin-like molecules are generated by snake venom metalloproteinases

Angiostatin is a plasminogen-derived anti-angiogenic factor composed of its first four kringle structures. This molecule is generated by proteolytic cleavage of plasminogen by some proteolytic enzymes in vitro. Since venoms of viper snakes are a rich source of both serine- and metalloproteinase, we hypothesized that angiostatin-like polypeptides could be generated during the envenomation after snake bites and play a pathophysiological role in the local tissue damage and regeneration. Our results showed that crude venoms from several species of Bothrops snakes were able to generate angiostatin-like polypeptides and purified metalloproteinases but not serine proteinases from Bothrops jararaca and Bothrops moojeni venoms were responsible for their generation in vitro. The putative plasminogen cleavage sites by the crude venoms and purified proteinases were determined by N-terminal amino acid sequencing of the angiostatin-like molecules. Angiostatin-like peptides derived from human plasminogen digestion by jararhagin, a metalloproteinase isolated from B. jararaca venom, inhibited endothelial cell proliferation in vitro. These results indicate that angiostatin-like molecules can be generated upon snakebite envenomations and may account for the poor and incomplete regenerative response observed in the damaged tissue.

Generation of biologically active angiostatin kringle 1-3 by activated human neutrophils

The contribution of polymorphonuclear neutrophils (PMN) to host defense and natural immunity extends well beyond their traditional role as professional phagocytes. In this study, we demonstrate that upon stimulation with proinflammatory stimuli, human PMN release enzymatic activities that, in vitro, generate bioactive angiostatin fragments from purified plasminogen. We also provide evidence that these angiostatin-like fragments, comprising kringle domain 1 to kringle domain 3 (kringle 1-3) of plasminogen, are generated as a byproduct of the selective proteolytic activity of neutrophil-secreted elastase. Remarkably, affinity-purified angiostatin kringle 1-3 fragments generated by neutrophils inhibited basic fibroblast growth factor plus vascular endothelial growth factor-induced endothelial cell proliferation in vitro, and both vascular endothelial growth factor-induced angiogenesis in the matrigel plug assay and fibroblast growth factor-induced angiogenesis in the chick embryo chorioallantoic membrane assay, in vivo. These results represent the first demonstration that biologically active angiostatin-like fragments can be generated by inflammatory human neutrophils. Because angiostatin is a potent inhibitor of angiogenesis, tumor growth, and metastasis, the data suggest that activated PMN not only act as potent effectors of inflammation, but might also play a critical role in the inhibition of angiogenesis in inflammatory diseases and tumors, by generation of a potent anti-angiogenic molecule.

Regulation of endothelial cell survival and apoptosis during angiogenesis

The process of angiogenesis plays an important role in many physiological and pathological conditions. Inhibition of endothelial cell (EC) apoptosis providing EC survival is thought to be an essential mechanism during angiogenesis. Many of the angiogenic growth factors inhibit EC apoptosis. In addition, the adhesion of ECs to the extracellular matrix or intercellular adhesion promotes EC survival. In contrast, increasing evidence suggests that the induction of EC apoptosis may counteract angiogenesis. In this review, we focus on the regulation of EC survival and apoptosis during angiogenesis and especially on the effects and intracellular signaling promoted by angiogenic growth factors, endogenous angiogenic inhibitors (such as angiostatin, endostatin, and thrombospondin-1), and the adhesion to the extracellular matrix. Furthermore, we discuss the effects of cross talk between adhesion molecules and growth factors. Understanding the molecular mechanisms involved in the regulation of EC survival and apoptosis may provide new targets for the development of new therapies to enhance angiogenesis in the case of tissue-ischemia (eg, the neovascularization of myocardium) or to inhibit angiogenesis in the case of neovascularization-dependent disease (eg, tumor, diabetic retinopathy).

Angiostatin II is the predominant glycoform in pleural effusates of rabbit VX-2 lung tumors

Angiostatin (AST), a polypeptide with potent antiangiogenic properties, is released proteolytically from plasminogen in vivo. Plasminogen exists naturally in plasma as two glycoforms (PLGs), I and II. Recently it was shown with the use of a chick-embryo chorioallantoic membrane (CAM) assay that rabbit PLG-I and -II yield distinct ASTs-AST-I and -II, respectively-with different antiangiogenic activities. AST glycoforms were of similar molecular weight, approximately 30 to 32,000 kD, and probably consisted of kringles 1 to 3 only. AST has now been identified in the interpleural effusate released from VX-2 lung tumors in rabbits. Effusate was collected from six rabbits with high tumor burdens and fractionated by means of lysine-Sepharose chromatography. The epsilon-aminohexanoic acid-eluted protein of all effusates contained AST (kringles 1-3) at a mean concentration of 1.2 microg/mL of effusate; with regard to AST content, 97% was AST-II. A CAM assay revealed that the lysine-Sepharose-bound fraction from all interpleural effusates contained potent antiangiogenic activity. Blood and urine from rabbits with high burdens of VX-2 contained essentially only AST-II, at mean concentrations of 145 and 4 ng/mL, respectively. AST was absent from the blood of control rabbits. In an attempt to compare their uptake by VX-2, iodine 125-labeled AST-I and iodine 131-labeled AST-II were injected intravenously into tumor-bearing rabbits. AST-I entered the tumor 1.6 times faster than AST-II. As a means of accounting for the preponderance of AST-II in the interpleural effusate, we postulate that VX-2 cells release proteolytic activity to activate plasminogen but that of the two PLGs, PLG-II may be the preferred substrate for AST formation in vivo.

The role of angiostatin in the spontaneous bone and prostate cancers of pet dogs

Angiostatin is a potent inhibitor of angiogenesis generated in cancer-bearing hosts by tumor-derived proteases. Because the naturally occurring bone and prostate cancers of pet dogs provide unique model systems to study factors that regulate cancer progression and tumor dormancy, we investigated the capacity of these tumors to generate angiostatin. We determined that angiostatin fragments are present in urine of dogs with bone cancer. The identity of these fragments was confirmed by comparison of the experimentally determined protein sequence to that of a clone of canine angiostatin. Importantly, these fragments were absent in urine collected from the same dogs after complete surgical removal of the primary tumor. We also demonstrate that canine prostate cancer cells are capable of processing plasminogen to angiostatin in vitro. These findings provide rationale for using spontaneous canine tumor models to isolate endogenous angiogenesis inhibitors and to investigate their therapeutic use against cancer.

Identification of annexin II heterotetramer as a plasmin reductase

Annexin II heterotetramer (AIIt) is a Ca(2+)- and phospholipid-binding protein that consists of two copies of a p36 and p11 subunit. AIIt regulates the production and autoproteolysis of plasmin at the cell surface. In addition to its role as a key cellular protease, plasmin also plays a role in angiogenesis as the precursor for antiangiogenic proteins. Recently we demonstrated that the primary antiangiogenic plasmin fragment, called A(61) (Lys(78)-Lys(468)) was released from cultured cells. In the present study we report for the first time that AIIt possesses an intrinsic plasmin reductase activity. AIIt stimulated the reduction of the plasmin Cys(462)-Cys(541) bond in a time- and concentration-dependent manner, which resulted in the release of A(61) from plasmin. Mutagenesis of p36 C334S and either p11 C61S or p11 C82S inactivated the plasmin reductase activity of the isolated subunits, suggesting that specific cysteinyl residues participated in the plasmin reductase activity of each subunit. Furthermore, we demonstrated that the loss of AIIt from the cell surface of HT1080 cells transduced with a retroviral vector encoding p11 antisense dramatically reduced the cellular production of A(61) from plasminogen. This is the first demonstration that AIIt regulates the cellular production of the antiangiogenic plasminogen fragment, A(61).

Plasmin reduction by phosphoglycerate kinase is a thiol-independent process

Phosphoglycerate kinase (PGK) is secreted by tumor cells and facilitates reduction of disulfide bond(s) in plasmin (Lay, A. J., Jiang, X.-M., Kisker, O., Flynn, E., Underwood, A., Condron, R., and Hogg, P. J. (2000) Nature 408, 869-873). The angiogenesis inhibitor, angiostatin, is cleaved from the reduced plasmin by a combination of serine- and metalloproteinases. The chemistry of protein reductants is typically mediated by a pair of closely spaced Cys residues. There are seven Cys in human PGK, and mutation of all seven to Ala did not appreciably affect plasmin reductase activity, although some of the mutations perturbed the tertiary structure of the protein. Cys-379 and Cys-380 are close to the hinge that links the N- and C-terminal domains of PGK. Alkylation/oxidation of Cys-379 and -380 by four different thiol-reactive compounds reduced plasmin reductase activity to 7--35% of control. Binding of 3-phosphoglycerate and/or MgATP to the N- and C-terminal domains of PGK, respectively, triggers a hinge bending conformational change in the enzyme. Incubation of PGK with 3-phosphoglycerate and/or MgATP ablated plasmin reductase activity, with half-maximal inhibitory effects at approximately 1 mm concentration. In summary, reduction of plasmin by PGK is a thiol-independent process, although either alkylation/oxidation of the fast-reacting Cys near the hinge or hinge bending conformational change in PGK perturbs plasmin reduction by PGK, perhaps by obstructing the interaction of plasmin with PGK or perturbing conformational changes in PGK required for plasmin reduction.

Tumor production of angiostatin is enhanced after exposure to TNF-alpha

Infection of tumors with an adenoviral vector expressing a chimeric gene composed of the CArG elements of the Egr-1 promoter and a cDNA encoding TNF-alpha (Ad.Egr-TNF) has previously been shown to result in the production of high intratumoral levels of TNF-alpha and thereby tumor regression. The antitumor effects of TNF-alpha were ascribed to vascular thrombosis. We and others, have reported that inhibition of tumor vessel thrombosis using anticoagulation therapy does not abrogate the antitumor effects after TNF-alpha treatment. To investigate the potential antiangiogenic effects of TNF-alpha, we studied the generation of angiostatin after intratumoral injection of Ad.Egr-TNF. We report an increase in plasma angiostatin levels both during and after treatment with Ad.Egr-TNF that parallel tumor regression. We also report that TNF-alpha enhances angiostatin production by inducing the activity of plasminogen activator and the release of MMP-9 by tumor cells. These studies support a model in which the antiangiogenic effects of TNF-alpha on the tumor microvasculature are mediated by generation of angiostatin.

Inhibition of spontaneous metastases formation by amifostine

Amifostine was investigated for its ability to inhibit spontaneous metastases formation using the well-characterized murine sarcoma, Sa-NH. Amifostine was administered intraperitoneally at a dose of 50 mg/kg every other day for 6 days to C3Hf/Kam mice until tumors reached an average size of 8-8.5 mm in diameter. Amifostine was again administered immediately after surgical removal of the tumor-bearing limbs by amputation, and then once more 2 days later. Twenty-one days later, animals were evaluated for the presence of spontaneously developed pulmonary metastases. Nontumor-bearing control animals were sham treated using the same dosing and surgery schedules. Treatment with amifostine appeared to slightly delay tumor growth, that is, 13 vs. 12 days for tumors to reach an average diameter of 8 mm. Amifostine reduced both the incidence of pulmonary metastases formed in experimental animals from 77% to 57% (p < 0.05), and their average number per animal from 12.8 +/- 5.4 (SEM) to 2.9 +/- 1.1 (SEM). The effect of amifostine exposure on serum levels of the angiogenesis inhibitor angiostatin was also determined using Western blot analysis. Consistent with the antimetastatic effect, exposure of animals to 50 mg/kg of amifostine resulted in a 4-fold enhanced serum level of angiostatin above control levels. This phenomenon occurred in tumor-bearing and nontumor-bearing animals. The effects of amifostine on matrix metalloproteinase (MMP) enzymatic activity was also determined using gelatin zymography. Conditioned growth medium collected from Sa-NH cells grown to confluency was exposed to various concentrations of SH, i.e., 2-[(aminopropyl)amino]ethane-thiol (WR-1065), the active thiol form of amifostine, for either 30 min or 18 hr. WR-1065, as a function of increasing dose and time, inhibited the enzymatic activities of MMP-2 and MMP-9. At a concentration and time of exposure likely to be achieved in vivo, that is, 40 microM and 30 min, MMP-2 and MMP-9 activities were reduced to between 30% and 40% of control values. Consistent with these affects, WR-1065 was also found to be effective in inhibiting the ability of Sa-NH cells to migrate through Matrigel membranes. After an 18-hr exposure under in vitro conditions, WR-1065 at concentrations of 4, 40 and 400 microM, and 4 mM, inhibited Sa-NH migration to 11%, 44%, 81% and 97% of control values, respectively. The abilities of amifostine and its active thiol WR-1065 to stimulate angiostatin production in mice, and to inhibit the MMP enzymatic activities and invasion ability of Sa-NH cells under in vitro conditions, are consistent with the observed antimetastatic effects exhibited against Sa-NH tumors growing in vivo.

p22 is a novel plasminogen fragment with antiangiogenic activity

Tumor or tumor-associated cells cleave circulating plasminogen into three or four kringle-containing antiangiogenic fragments, collectively referred to as angiostatin. Angiostatin blocks tumor growth and metastasis by preventing the growth of endothelial cells that are critical for tumor vascularization. Here, we show that cancer and normal cells convert plasminogen into a novel 22 kDa fragment (p22). Production of this plasminogen fragment in a cell-free system has allowed characterization of the structure and activity of the protein. p22 consists of amino acid residues 78-180 of plasminogen and therefore embodies the first plasminogen kringle (residues 84-162) as well as additional N- and C-terminal residues. Circular dichroism and intrinsic fluorescence spectrum analysis have defined structural differences between p22 and recombinant plasminogen kringle 1 (rK1), therefore suggesting a unique conformation for kringle 1 within p22. Proliferation of capillary endothelial cells but not cells of other lineages was selectively inhibited by p22 in vitro. In addition, p22 prevented vascular growth of chick chorioallantoic membranes (CAMs) in vivo. Furthermore, administration of p22 at low dose suppressed the growth of murine Lewis lung carcinoma (LLC) metastatic foci in vivo. This is the first identification of a single kringle-containing antiangiogenic plasminogen fragment produced under physiological conditions.

Adenoviral vector expressing murine angiostatin inhibits a model of breast cancer metastatic growth in the lungs of mice

Angiostatin, an internal fragment of plasminogen, has been shown to inhibit the process of angiogenesis or neovascularization. In this study, we have expressed the cDNA for murine angiostatin under the control of the human cytomegalovirus promoter from a human type-5 adenovirus and shown that this vector produces a protein which retains biological activity. Angiostatin expression was determined by Northern blot analysis and Western immunoblotting. Ad-angiostatin, but not a control vector Ad-dl70, significantly reduced the viability of infected human umbilical cord vein endothelial cells (HUVEC) in vitro. In an in vivo model of basic fibroblast growth factor-induced angiogenesis, Ad-angiostatin (1 x 10(9) pfu) could inhibit endothelial cell migration and the formation of capillaries within a Matrigel plug which had been implanted for one week subcutaneously into C57BL/6 mice. Endothelial cells in these plugs had an altered, rounded, phenotype with dark picnotic nuclei indicative of apoptosis, which was confirmed using transmission electron microscopy. In contrast, endothelial cells from bFGF alone or in combination with the control vector-treated plugs retained the long spindle shape characteristic of endothelial cells. Intranasal delivery of Ad-angiostatin into the lungs of FVB/n mice demonstrated comparable cellular infiltration in the recovered bronchoalveolar lavage fluid with no signs of abnormal pathology as compared to PBS or control vector-treated animals. In a pulmonary metastatic breast cancer model, the delivery of Ad-angiostatin (1 x 10(9) pfu) to the lung significantly delayed tumor growth as measured by the number of visible surface tumor nodules. This study has demonstrated that the specific targeting of tumors to inhibit angiogenesis using an adenovirus expressing angiostatin, may deliver localized concentrations of protein having a greater impact on inhibition of tumor growth.

Metabolism of rabbit angiostatin glycoforms I and II in rabbits: angiostatin-I leaves the intravascular space faster and appears to have greater anti-angiogenic activity than angiostatin-II

Plasminogen (PLG) exists in the circulation as two glycoforms, I and II. Angiostatin (AST) is a polypeptide that has been cleaved from the kringle region of PLG and has strong anti-angiogenic properties. AST-I and AST-II, which consisted only of kringles 1 through 3, were prepared by the action of urokinase on purified rabbit PLG-I and PLG-II, respectively, in the presence of N-acetyl cysteine, followed by affinity chromatography on lysine-Sepharose. Purified AST-I and AST-II were tested for functional activity with a chick chorioallantoic membrane (CAM) model; when similar amounts were applied to a 6-day CAM, AST-I was substantially more effective than AST-II in decreasing vascular supply to the CAM over a 72-hour period; this activity correlated with a loss of capillaries, probably through apoptosis of endothelial cells. Radiolabeled AST-I and AST-II (iodine 125 and iodine 131) were co-injected intravenously into healthy rabbits to determine their clearances from plasma measured over 3 days. Over a dose range of 0.08 to 2.7 microg/kg, the fractional catabolic rate within the intravascular space (j(3)) indicated that AST-I was cleared 3-fold to 4-fold more rapidly than AST-II (P < .001). The catabolic half-life of AST-I (2.01 +/- 0.19 days) was significantly less than that of AST-II (2.62 +/- 0.20 days). The faster clearance of AST-I from the intravascular space was matched by its more rapid passage than AST-II to the extravascular space of various organs over 60 minutes in vivo. This property of AST-I as compared with AST-II may partially explain its greater anti-angiogenic potential. From the plasma concentrations of PLG-I and PLG-II and their relative behaviors toward rabbit VX-2 lung tumors in vivo, we predict that substantially greater quantities of AST-II than AST-I may be released into the extravascular space of tumors.

Expression and localisation of thioredoxin in mouse reproductive tissues during the oestrous cycle

Thioredoxin expression within the reproductive tissues of the female mouse was analysed during the oestrous cycle stages of dioestrus, oestrus and metoestrus by Western blot analyses and immunocytochemistry. From Western blot analyses the expression of thioredoxin was found to be increased in oestrus compared to dioestrus and metoestrus. Localisation of thioredoxin within the reproductive organs of the mouse during the oestrous cycle has shown that the expression of thioredoxin is specific for distinct areas within the reproductive organs. These areas are the stratified squamous epithelium of the vagina, the simple columnar epithelium and the uterine glands of the uterus, the ciliated columnar epithelium of the oviduct, the corpus lutea, the interstitial cells and the secondary follicles of the ovary. The discrete cellular localisation and oestrous dependence of thioredoxin expression are suggestive of specific roles in various reproductive processes.

Matrix metalloproteinase deficiencies do not impair cell-associated fibrinolytic activity

The matrix metalloproteinase (MMP) and fibrinolytic (plasminogen/plasmin) systems cooperate in many (patho)physiological processes requiring extracellular proteolysis. The effect of MMP-3 (stromelysin-1), MMP-7 (matrilysin), MMP-9 (gelatinase B) or MMP-12 (metalloelastase) on cellular fibrinolytic activity was studied with the use of smooth muscle cells (SMC) and fibroblasts derived from mice with specific inactivation of these genes. Activation of cell-bound plasminogen by two-chain urokinase-type plasminogen activator (tcu-PA) was not significantly different with SMC or fibroblasts from the gene-deficient mice (78% to 140% of wild-type). For all cell types, very limited conversion of plasminogen to angiostatin-like kringle-containing fragments was observed (< 3% of the total cell-bound plasminogen). Activation of plasminogen in solution by cell-associated tcu-PA was also comparable for SMC or fibroblasts of the different genotypes (54% to 160% of wild-type). In vitro SMC migration on scrape wounded collagen-coated surfaces was comparable for wild-type, MMP-7(-/-), MMP-9(-/-) and MMP-12(-/-) SMC, but was significantly reduced for MMP-3(-/-) SMC (P < .005 vs. wild-type). Serum-free conditioned medium of MMP-3(-/-) and MMP-7(-/-) SMC or fibroblasts induced similar lysis of fibrin films as wild-type cells. These findings indicate that several interactions that have been described between these MMPs and the plasminogen/plasmin system in a purified system do not significantly affect plasmin-mediated cellular fibrinolytic activity under cell culture conditions.

Enzyme-linked immunosorbent assay for the specific detection of angiostatin-like plasminogen moieties in biological samples

An enzyme-linked immunosorbent assay (ELISA) was developed for the specific detection of human angiostatin-like plasminogen moieties (comprising kringles 1-4) in biological samples. The assay involves prior removal of all other plasminogen moieties by immunoadsorption of diluted samples (to about 10 ng/ml plasminogen) with a mixture of insolubilized MA-42B12 (directed against kringle 5) and MA-31E9 (directed against the proteinase domain). The recovery of angiostatin during this procedure is > or = 95%. Subsequently, angiostatin-like fragments are detected in an ELISA, based on two monoclonal antibodies reacting with nonoverlapping epitopes in the kringle 1-3 domain: MA-36E6 for capture and MA-34D3 for tagging. The assay has a lower detection limit of about 0.1 ng/ml and is performed with intra- and interassay coefficient of variation of 2.4% and 15%. In tumor fluids obtained from cancer patients (n = 10), angiostatin levels ranged between 0.24 and 6.7 microg/ml (1.62+/-0.60 microg/ml; mean+/-S.E.M.) The identity of angiostatin was confirmed by immunoblotting using specific monoclonal antibodies. A weak correlation (r = .66) was observed with the total plasminogen concentration in these samples. This ELISA thus appears suitable for the specific quantitation of angiostatin-like plasminogen moieties in biological samples, and may be useful to study its (patho)physiological relevance.

Purification and characterization of A61. An angiostatin-like plasminogen fragment produced by plasmin autodigestion in the absence of sulfhydryl donors

Plasmin, a broad spectrum proteinase, is inactivated by an autoproteolytic reaction that results in the destruction of the heavy and light chains of the protein. Recently we demonstrated that a 61-kDa plasmin fragment was one of the major products of this autoproteolytic reaction (Fitzpatrick, S. L., Kassam, G., Choi, K. S., Kang, H. M., Fogg, D. K., and Waisman, D. M. (2000)Biochemistry 39, 1021-1028). In the present communication we have identified the 61-kDa plasmin fragment as a novel four kringle-containing protein consisting of the amino acid sequence Lys(78)-Lys(468). To avoid confusion with the plasmin(ogen) fragment, angiostatin(R) (Lys(78)-Ala(440)), we have named this protein A(61). Unlike angiostatin, A(61) was produced in vitro from plasmin autodigestion in the absence of sulfhydryl donors. A(61) bound to lysine-Sepharose and also underwent a large increase in fluorescence yield upon binding of the lysine analogue, trans-4-aminomethylcyclohexanecarboxylic acid. Circular dichroism suggested that A(61) was composed of 21% beta-strand, 14% beta-turn, 18% 3(1)-helix and 8% 3(10)-helix. A(61) was an anti-angiogenic protein as indicated by the inhibition of bovine capillary endothelial cell proliferation. Plasminogen was converted to A(61) by HT1080 cells and bovine capillary endothelial cells. Furthermore, a plasminogen fragment similar to A(61) was present in the serum of humans as well as normal and tumor-bearing mice. These results establish that plasmin turnover can generate anti-angiogenic plasmin fragments in a nonpathological setting.

A truncated plasminogen activator inhibitor-1 protein induces and inhibits angiostatin (kringles 1-3), a plasminogen cleavage product

Plasminogen activator inhibitor-1 (PAI-1) is a serpin protease inhibitor that binds plasminogen activators (uPA and tPA) at a reactive center loop located at the carboxyl-terminal amino acid residues 320-351. The loop is stretched across the top of the active PAI-1 protein maintaining the molecule in a rigid conformation. In the latent PAI-1 conformation, the reactive center loop is inserted into one of the beta sheets, thus making the reactive center loop unavailable for interaction with the plasminogen activators. We truncated porcine PAI-1 at the amino and carboxyl termini to eliminate the reactive center loop, part of a heparin binding site, and a vitronectin binding site. The region we maintained corresponds to amino acids 80-265 of mature human PAI-1 containing binding sites for vitronectin, heparin (partial), uPA, tPA, fibrin, thrombin, and the helix F region. The interaction of "inactive" PAI-1, rPAI-1(23), with plasminogen and uPA induces the formation of a proteolytic protein with angiostatin properties. Increasing amounts of rPAI-1(23) inhibit the proteolytic angiostatin fragment. Endothelial cells exposed to exogenous rPAI-1(23) exhibit reduced proliferation, reduced tube formation, and 47% apoptotic cells within 48 h. Transfected endothelial cells secreting rPAI-1(23) have a 30% reduction in proliferation, vastly reduced tube formation, and a 50% reduction in cell migration in the presence of VEGF. These two studies show that rPAI-1(23) interactions with uPA and plasminogen can inhibit plasmin by two mechanisms. In one mechanism, rPAI-1(23) cleaves plasmin to form a proteolytic angiostatin-like protein. In a second mechanism, rPAI-1(23) can bind uPA and/or plasminogen to reduce the number of uPA and plasminogen interactions, hence reducing the amount of plasmin that is produced.

Proteinase-activated receptor-1 regulation of macrophage elastase (MMP-12) secretion by serine proteinases

The serine proteinases plasmin and thrombin convert proenzyme matrix metalloproteinases (MMPs) into catalytically active forms. In addition, we demonstrate that plasmin(ogen) and thrombin induce a significant increase in secretion of activated murine macrophage elastase (MMP-12) protein. Active serine protease is responsible for induction, as demonstrated by the absence of MMP-12 induction in plasminogen(Plg)-treated urokinase-type plasminogen activator-deficient macrophages. Since increased MMP-12 protein secretion was not accompanied by an increase in MMP-12 mRNA, we examined post-translational mechanisms. Protein synthesis was not required for early release of MMP-12 but was required for later secretion of activated enzyme. Immunofluorescent microscopy demonstrated basal expression in macrophages that increased following serine proteinase exposure. Inhibition of MMP-12 secretion by hirudin and pertussis toxin demonstrated a role for the thrombin G protein-coupled receptor (protease-activated receptor 1 (PAR-1)). PAR-1-activating peptides were able to induce MMP-12 release. Investigation of signal transduction pathways involved in this response demonstrate the requirement for protein kinase C, but not tyrosine kinase, activity. These data demonstrate that plasmin and thrombin regulate MMP-12 activity through distinct mechanisms: post-translational secretion of preformed MMP-12 protein, induction of protein secretion that is protein kinase C-mediated, and extracellular enzyme activation. Most importantly, we show that serine proteinase MMP-12 regulation in macrophages occurs via the protein kinase C-activating G protein-coupled receptor PAR-1.

Phosphoglycerate kinase acts in tumour angiogenesis as a disulphide reductase

Disulphide bonds in secreted proteins are considered to be inert because of the oxidizing nature of the extracellular milieu. An exception to this rule is a reductase secreted by tumour cells that reduces disulphide bonds in the serine proteinase plasmin. Reduction of plasmin initiates proteolytic cleavage in the kringle 5 domain and release of the tumour blood vessel inhibitor angiostatin. New blood vessel formation or angiogenesis is critical for tumour expansion and metastasis. Here we show that the plasmin reductase isolated from conditioned medium of fibrosarcoma cells is the glycolytic enzyme phosphoglycerate kinase. Recombinant phosphoglycerate kinase had the same specific activity as the fibrosarcoma-derived protein. Plasma of mice bearing fibrosarcoma tumours contained several-fold more phosphoglycerate kinase, as compared with mice without tumours. Administration of phosphoglycerate kinase to tumour-bearing mice caused an increase in plasma levels of angiostatin, and a decrease in tumour vascularity and rate of tumour growth. Our findings indicate that phosphoglycerate kinase not only functions in glycolysis but is secreted by tumour cells and participates in the angiogenic process as a disulphide reductase.

Angiostatin generation by cathepsin D secreted by human prostate carcinoma cells

Angiostatin, a potent endogenous inhibitor of angiogenesis, is generated by cancer-mediated proteolysis of plasminogen. The culture medium of human prostate carcinoma cells, when incubated with plasminogen at a variety of pH values, generated angiostatic peptides and miniplasminogen. The enzyme(s) responsible for this reaction was purified and identified as procathepsin D. The purified procathepsin D, as well as cathepsin D, generated two angiostatic peptides having the same NH(2)-terminal amino acid sequences and comprising kringles 1-4 of plasminogen in the pH range of 3.0-6.8, most strongly at pH 4.0 in vitro. This reaction required the concomitant conversion of procathepsin D to catalytically active pseudocathepsin D. The conversion of pseudocathepsin D to the mature cathepsin D was not observed by the prolonged incubation. The affinity-purified angiostatic peptides inhibited angiogenesis both in vitro and in vivo. Importantly, procathepsin D secreted by human breast carcinoma cells showed a significantly lower angiostatin-generating activity than that by human prostate carcinoma cells. Since deglycosylated procathepsin D from both prostate and breast carcinoma cells exhibited a similar low angiostatin-generating activity, this discrepancy appeared to be attributed to the difference in carbohydrate structures of procathepsin D molecules between the two cell types. The seminal vesicle fluid from patients with prostate carcinoma contained the mature cathepsin D and procathepsin D, but not pseudocathepsin D, suggesting that pseudocathepsin D is not a normal intermediate of procathepsin D processing in vivo. The present study provides evidence for the first time that cathepsin D secreted by human prostate carcinoma cells is responsible for angiostatin generation, thereby causing the prevention of tumor growth and angiogenesis-dependent growth of metastases.

Matrix metalloproteinases: pro- and anti-angiogenic activities

Matrix metalloproteinases (MMP) are a family of structurally related proteinases most widely recognized for their ability to degrade extracellular matrix, although recent investigations have demonstrated other biologic functions for these enzymes. MMP are typically not constitutively expressed, but are regulated by: (1) cytokines, growth factors, and cell-cell and cell-matrix interactions that control gene expression; (2) activation of their proenzyme form; and (3) the presence of MMP inhibitors [tissue inhibitors of metalloproteinases, (TIMP)]. MMP have important roles in normal processes including development, wound healing, mammary gland, and uterine involution, but are also involved in angiogenesis, tumor growth, and metastasis. Angiogenesis, characteristically defined as the establishment of new vessels from pre-existing vasculature, is required for biologic processes such as wound healing and pathologic processes such as arthritis, tumor growth, and metastasis. Blocking of MMP activity has been studied for potential therapeutic efficacy in controlling such pathologic processes. Synthetic MMP inhibitors, most notably the hydroxymates, have been engineered for this purpose and are presently in clinical trial. These inhibitors may have broad versus specific MMP inhibitory activity. As increased non-matrix degrading capabilities of MMP are recognized, however, i.e., cytokine activation, processing of proteins to molecules of distinct biologic function, it becomes less clear whether the nonselective inhibition of MMP activity for all pathologic processes involving MMP is appropriate. This review focuses upon the contribution of MMP to the process of tumor invasion and angiogenesis, and discusses the design and use of MMP inhibitors as therapeutic agents in these processes.

New concepts in fibrinolysis and angiogenesis

Angiogenesis, the process by which new blood vessels form from preexisting vasculature, underlies a number of biologic processes including embryologic development, inflammation, wound healing, hypoxic retinal vascular proliferation, tumor growth, and atherosclerosis. The fibrinolytic system represents a cascade of serine protease activation events that culminate in the generation of plasmin. Although in-vitro studies suggest several possible roles that plasmin might play in angiogenesis, angiogenesis and fibrinolytic activity do not always correlate in in-vivo systems. During cutaneous and corneal wound healing, for example, angiogenesis proceeds normally in plasminogen-deficient animals. Similarly, the growth of most neoplasms is unimpaired in the absence of plasminogen. On the other hand, hypoxia-driven vascular proliferation may require plasmin-like activity, and angiogenesis within the atherosclerotic plaque seems to be associated with increased expression of fibrinolytic proteins. Recently, several nonplasmin fibrinolysins that may support the invasive phenotype of endothelial cells under specific circumstances have been identified. Thus, the contribution of individual fibrinolysins appears to be context-specific, just as the profile of endothelial cell gene expression depends upon the surrounding tissue milieu.

No grip, no growth: the conceptual basis of excessive proteolysis in the treatment of cancer

The formation of new bloodvessels, called angiogenesis, is critical for a tumour to grow beyond a few mm(3) in size. A provisional matrix promotes endothelial cell adhesion, migration, proliferation and survival. Synthesis and degradation of this matrix closely resemble processes that occur during coagulation and fibrinolysis. Degradation of the matrix and fibrinolysis are tightly controlled and balanced by stimulators and inhibitors of the plasminogen activation system. Here we give an overview of these processes during tumour progression. We postulate a novel way to inhibit angiogenesis by removal of the matrix through specific and localised overstimulation of the plasminogen activation system.

Disruption of interkringle disulfide bond of plasminogen kringle 1-3 changes the lysine binding capability of kringle 2, but not its antiangiogenic activity

Kringle 1-3 of human plasminogen is a potent inhibitor of endothelial cell proliferation. To understand a possible role for the unique cystine bridge between kringle 2 and kringle 3, we disrupted the interkringle disulfide bond by mutating Cys(169) and Cys(297) to serine residues. The yield of the mutant during the refolding process was decreased significantly. Anti-endothelial cell proliferative activity of the mutant was similar to that of the wild type. There was no significant difference in in vivo antiangiogenic activity between the wild type and the mutant in chorioallantoic membrane assay. However, in the mutant, the weak lysine binding capability of kringle 2 was not detected and its mobility in nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis is different from that of the wild type. These results support the notion that the overall antiangiogenic function of angiostatin is mediated by individual kringles, and suggest that the lysine binding capability of kringle 2 is likely not important for the antiangiogenic activity of kringle 1-3.

Protein-disulfide isomerase (PDI) in FRTL5 cells. pH-dependent thyroglobulin/PDI interactions determine a novel PDI function in the post-endoplasmic reticulum of thyrocytes

Thyroglobulin (TG) is secreted by the thyrocytes into the follicular lumen of the thyroid. After maturation and hormone formation, TG is endocytosed and delivered to lysosomes. Quality control mechanisms may occur during this bidirectional traffic since 1) several molecular chaperones are cosecreted with TG in vivo, and 2) lysosomal targeting of immature TG is thought to be prevented via the interaction, in acidic conditions, between the Ser(789)-Met(1172) TG hormonogenic domain (BD) and an unidentified membrane receptor. We investigated the secretion and cell surface expression of PDI and other chaperones in the FRTL5 thyroid cell line, and then studied the characteristics of the interaction between TG and PDI. We demonstrated that PDI, but also other chaperones such as calnexin and KDEL-containing proteins are exposed at the cell surface. We observed on living cells or membrane preparations that PDI specifically binds TG in acidic conditions, and that only BD is involved in binding. Surface plasmon resonance analysis of TG/PDI interactions indicated: 1) that PDI bound TG but only in acidic conditions, and that it preferentially recognized immature molecules, and 2) BD is involved in binding even if cysteine-rich modules are deleted. The notion that PDI acts as an "escort" for immature TG in acidic post-endoplasmic reticulum compartments is discussed.

Specific proteolysis of human plasminogen by a 24 kDa endopeptidase from a novel Chryseobacterium Sp

A novel single polypeptide endopeptidase of 24 kDa (24k-endopeptidase) was purified with a yield of 300-400 microg/L from conditioned medium of a bacterial strain which was identified as a new species in the genus Chryseobacterium Sp. on the basis of its 16S rDNA sequence and DNA:DNA hybridizations. The NH(2)-terminal amino acid sequence (Val-Ala-Thr-Pro-Asn-Leu-Glu-.) was not found in the availabe databases. The 24k-endopeptidase specifically hydrolyzed the Ser(441)-Val(442) peptide bond in human plasmin(ogen), with additional cleavage of the Lys(78)-Val(79) and Pro(447)-Val(448) peptide bonds, and a secondary cleavage at Lys(615)-Val(616). Thereby, plasminogen is converted into an angiostatin-like fragment containing kringles 1-4 (K1-4) and miniplasminogen (kringle 5 and the serine proteinase domain). The purified K1-4 fragment showed a comparable cytotoxicity toward endothelial cells as the elastase-derived K1-3 fragment (12.7% versus 10.6% at a concentration of 10 microg/mL). Plasminogen, bound to monocytoid THP-1 cells, was also cleaved by the 24k-endopeptidase, resulting in generation of an angiostatin-like fragment and in a decreased capacity to generate cell-associated plasmin following activation by urokinase. The 24k-endopeptidase was not efficiently neutralized by specific inhibitors against the serine, cysteine, aspartic, or matrix metalloproteinase classes of enzymes. In human plasma or serum, however, it induced only very limited plasminogen degradation, apparently due to neutralization of its activity by alpha(2)-macroglobulin. Interaction of this novel 24k-endopeptidase with plasminogen thus yields an angiostatin-like fragment and affects plasmin-mediated cellular proteolytic activity.

Redox control of protein degradation

This review summarizes evidence that most of cell protein degradation is maintained by pathways transferring energy from glucose to reduction of enzymic and nonenzymic proteins (redox-responsive). In contrast, a major subcomponent of proteolysis is simultaneously independent of the cell redox network (redox-unresponsive). Thus far, direct and indirect redox-responsive proteolytic effector mechanisms characterized by various investigators include: several classes of proteases, some peptide protease inhibitors, substrate conjugation systems, substrate redox and folding status, cytoskeletal-membrane kinesis, metal homeostasis, and others. The present focus involves redox control of sulfhydryl proteases and proteolytic pathways of mammalian muscle; however, other mechanisms, cell types, and species are also surveyed. The diversity of redox-responsive catabolic mechanisms reveals that the machinery of protein turnover evolved with fundamental dependencies upon the cell redox network, as observed in many species. The net redox status of a reversible proteolytic effector mechanism represents the balance between combined oxidative inactivating influences versus reductive activating influences. Similar to other proteins, redox-responsive proteolytic effectors appear to be oxidized by mixed disulfide formation, nitrosation, reactive oxygen species, and associations or reactions with metal ions and various pro-oxidative metabolites. Systems reducing the proteolytic machinery include major redox enzyme chains, such as thioredoxins or glutaredoxins, and perhaps various reductive metabolites, including glutathione and dihydrolipoic acid. Much of mammalian intracellular protein degradation is reversibly responsive to noninjurious experimental intervention in the reductive energy supply-demand balance. Proteolysis is reversibly inhibited by diamide or dehydroascorbic acid; and such antiproteolytic actions are strongly dependent on the cell glucose supply. However, gross redox-responsive proteolysis is not accompanied by ATP depletion or vice versa. Redox-responsive proteolysis includes Golgi-endoplasmic reticulum degradation, lysosomal degradation, and some amount of extravesicular degradation, all comprising more than half of total cell proteolysis. Speculatively, redox-dependent proteolysis exhibits features expected of a controlling influence coordinating distinct proteolytic processes under some intracellular conditions.

The tumor-suppressing activity of angiostatin protein resides within kringles 1 to 3

Angiostatin protein, which comprises the first four kringle domains of plasminogen, is an endogenous inhibitor of angiogenesis that inhibits the growth of experimental primary and metastatic tumors. Truncation of Angiostatin K1-4 to K1-3 retained the activity of Angiostatin. We recombinantly expressed full-length human Angiostatin protein corresponding to the first four kringle domains of human plasminogen and a truncated form of the Angiostatin protein, kringles 1-3. Purified recombinant Angiostatin K1-3 and K1-4 proteins inhibited the formation of experimental B16-BL6 lung metastases by greater than 80% when administered at 30 nmol/kg/day. We demonstrate for the first time that Angiostatin protein, consisting of the first three kringle domains of human plasminogen, has in vivo biological activity in this assay indistinguishable from that of the full-length Angiostatin K1-4 protein and that the fourth kringle of plasminogen, when linked in sequence to K1-3, plays no direct role in the antitumor activity of Angiostatin.

Regulation of angiostatin production by matrix metalloproteinase-2 in a model of concomitant resistance

We have previously reported the identification of the endogenous angiogenesis inhibitor angiostatin, a specific inhibitor of endothelial cell proliferation in vitro and angiogenesis in vivo. In our original studies, we demonstrated that a Lewis lung carcinoma (LLC-LM) primary tumor could suppress the growth of its metastases by generating angiostatin. Angiostatin, a 38-kDa internal fragment of plasminogen, was purified from the serum and urine of mice bearing LLC-LM, and its discovery provides the first proven mechanism for concomitant resistance (O'Reilly, M. S., Holmgren, L., Shing, Y., Chen, C., Rosenthal, R. A., Moses, M. A., Lane, W. S., Cao, Y., Sage, E. H., and Folkman, J. (1994) Cell 79, 315-328). Subsequently, we have shown that systemic administration of angiostatin can regress a wide variety of malignant tumors in vivo. However, at the time of our initial discovery of angiostatin, the source of the protein was unclear. We hypothesized that the tumor or stromal cells might produce an enzyme that could cleave plasminogen sequestered by the primary tumor into angiostatin. Alternatively, we speculated that the tumor cells might express angiostatin. By Northern analysis, however, we have found no evidence that the tumor cells express angiostatin or other fragments of plasminogen (data not shown). We now report that gelatinase A (matrix metalloproteinase-2), produced directly by the LLC-LM cells, is responsible for the production of angiostatin, which suppresses the growth of metastases in our original model.

Angiostatin inhibits endothelial and melanoma cellular invasion by blocking matrix-enhanced plasminogen activation

Angiostatin, a kringle-containing fragment of plasminogen, is a potent inhibitor of angiogenesis. The mechanism(s) responsible for the anti-angiogenic properties of angiostatin are unknown. We now report that human angiostatin blocks plasmin(ogen)-enhanced in vitro invasion of tissue plasminogen activator (t-PA)-producing endothelial and melanoma cells. Kinetic analyses demonstrated that angiostatin functions as a non-competitive inhibitor of extracellular-matrix (ECM)-enhanced, t-PA-catalysed plasminogen activation, with a Ki of 0.9+/-0.03 microM. This mechanism suggests that t-PA has a binding site for the inhibitor angiostatin, as well as for its substrate plasminogen that, when occupied, prevents ternary complex formation between t-PA, plasminogen and matrix protein. Direct binding experiments confirmed that angiostatin bound to t-PA with an apparent Kd [Kd(app)] of 6.7+/-0.7 nM, but did not bind with high affinity to ECM proteins. Together, these data suggest that angiostatin in the cellular micro-environment can inhibit matrix-enhanced plasminogen activation, resulting in reduced invasive activity, and suggest a biochemical mechanism whereby angiostatin-mediated regulation of plasmin formation could influence cellular migration and invasion.

Angiogenesis activators and inhibitors differentially regulate caveolin-1 expression and caveolae formation in vascular endothelial cells. Angiogenesis inhibitors block vascular endothelial growth factor-induced down-regulation of caveolin-1

Angiogenesis is the process by which new blood vessels are formed via proliferation of vascular endothelial cells. A variety of angiogenesis inhibitors that antagonize the effects of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) have recently been identified. However, the mechanism by which these diverse angiogenesis inhibitors exert their common effects remains largely unknown. Caveolin-1 and -2 are known to be highly expressed in vascular endothelial cells both in vitro and in vivo. Here, we examine the potential role of caveolins in the angiogenic response. For this purpose, we used the well established human umbilical vein endothelial cell line, ECV 304. Treatment of ECV 304 cells with known angiogenic growth factors (VEGF, bFGF, or hepatocyte growth factor/scatter factor), resulted in a dramatic reduction in the expression of caveolin-1. This down-regulation event was selective for caveolin-1, as caveolin-2 levels remained constant under these conditions of growth factor stimulation. VEGF-induced down-regulation of caveolin-1 expression also resulted in the morphological loss of cell surface caveolae organelles as seen by transmission electron microscopy. A variety of well characterized angiogenesis inhibitors (including angiostatin, fumagillin, 2-methoxy estradiol, transforming growth factor-beta, and thalidomide) effectively blocked VEGF-induced down-regulation of caveolin-1 as seen by immunoblotting and immunofluorescence microscopy. However, treatment with angiogenesis inhibitors alone did not significantly affect the expression of caveolin-1. PD98059, a specific inhibitor of mitogen-activated protein kinase and a known angiogenesis inhibitor, also blocked the observed VEGF-induced down-regulation of caveolin-1. Furthermore, we show that caveolin-1 can function as a negative regulator of VEGF-R (KDR) signal transduction in vivo. Thus, down-regulation of caveolin-1 may be an important step along the pathway toward endothelial cell proliferation.

Suppression of angiogenesis and tumor growth by the inhibitor K1-5 generated by plasmin-mediated proteolysis

Proteolytic enzymes are involved in generation of a number of endogenous angiogenesis inhibitors. Previously, we reported that angiostatin, a potent angiogenesis inhibitor, is a proteolytic fragment containing the first four kringle modules of plasminogen. In this report, we demonstrate that urokinase-activated plasmin can process plasminogen to release an angiogenesis inhibitor, K1-5 (protease-activated kringles 1-5). K1-5 inhibits endothelial-cell proliferation with a half-maximal concentration of approximately 50 pM. This inhibitory effect is endothelial-cell-specific and appears to be at least approximately 50-fold greater than that of angiostatin. A synergistic efficacy of endothelial inhibition was observed when angiostatin and kringle 5 (K5) were coincubated with capillary endothelial cells. The synergistic effect is comparable to that produced by K1-5 alone. Systemic treatment of mice with K1-5 at a low dose significantly blocked the fibroblast growth factor-induced corneal neovascularization, whereas angiostatin had no effect at the same dose. K1-5 also suppressed angiogenesis in chicken embryos. Systemic administration of K1-5 at a low dose at which angiostatin was ineffective significantly suppressed the growth of a murine T241 fibrosarcoma in mice. The antitumor effect correlates with the reduced neovascularization. These findings suggest that the plasmin-mediated proteolysis may be involved in the negative switch of angiogenesis.

Angiostatin formation involves disulfide bond reduction and proteolysis in kringle 5 of plasmin

Plasmin is processed in the conditioned medium of HT1080 fibrosarcoma cells producing fragments with the domain structures of the angiogenesis inhibitor, angiostatin, and microplasmin. Angiostatin consists of kringle domains 1-4 and part of kringle 5, while microplasmin consists of the remainder of kringle 5 and the serine proteinase domain. Our findings indicate that formation of angiostatin/microplasmin involves reduction of plasmin by a plasmin reductase followed by proteolysis of the reduced enzyme. We present evidence that the Cys461-Cys540 and Cys511-Cys535 disulfide bonds in kringle 5 of plasmin were reduced by plasmin reductase. Plasmin reductase activity was secreted by HT1080 and Chinese hamster ovary cells and the human mammary carcinoma cell lines MCF-7, MDA231, and BT20 but not by the monocyte/macrophage cell line THP-1. Neither primary foreskin fibroblasts, blood monocyte/macrophages, nor macrovascular or microvascular endothelial cells secreted detectable plasmin reductase. In contrast, cultured bovine and rat vascular smooth muscle cells secreted small but reproducible levels of plasmin reductase. Reduction of the kringle 5 disulfide bonds triggered cleavage at either Arg529-Lys530 or two other positions C-terminal of Cys461 in kringle 5 by a serine proteinase. Plasmin autoproteolysis could account for the cleavage, although another proteinase was mostly responsible in HT1080 conditioned medium. Three serine proteinases with apparent Mr of 70, 50, and 39 were purified from HT1080 conditioned medium, one or more of which could contribute to proteolysis of reduced plasmin.

Angiostatin binds ATP synthase on the surface of human endothelial cells

Angiostatin, a proteolytic fragment of plasminogen, is a potent antagonist of angiogenesis and an inhibitor of endothelial cell migration and proliferation. To determine whether the mechanism by which angiostatin inhibits endothelial cell migration and/or proliferation involves binding to cell surface plasminogen receptors, we isolated the binding proteins for plasminogen and angiostatin from human umbilical vein endothelial cells. Binding studies demonstrated that plasminogen and angiostatin bound in a concentration-dependent, saturable manner. Plasminogen binding was unaffected by a 100-fold molar excess of angiostatin, indicating the presence of a distinct angiostatin binding site. This finding was confirmed by ligand blot analysis of isolated human umbilical vein endothelial cell plasma membrane fractions, which demonstrated that plasminogen bound to a 44-kDa protein, whereas angiostatin bound to a 55-kDa species. Amino-terminal sequencing coupled with peptide mass fingerprinting and immunologic analyses identified the plasminogen binding protein as annexin II and the angiostatin binding protein as the alpha/beta-subunits of ATP synthase. The presence of this protein on the cell surface was confirmed by flow cytometry and immunofluorescence analysis. Angiostatin also bound to the recombinant alpha-subunit of human ATP synthase, and this binding was not inhibited by a 2,500-fold molar excess of plasminogen. Angiostatin's antiproliferative effect on endothelial cells was inhibited by as much as 90% in the presence of anti-alpha-subunit ATP synthase antibody. Binding of angiostatin to the alpha/beta-subunits of ATP synthase on the cell surface may mediate its antiangiogenic effects and the down-regulation of endothelial cell proliferation and migration.

Redox control of exofacial protein thiols/disulfides by protein disulfide isomerase

Protein disulfide isomerase (PDI) facilitates proper folding and disulfide bonding of nascent proteins in the endoplasmic reticulum and is secreted by cells and associates with the cell surface. We examined the consequence of over- or underexpression of PDI in HT1080 fibrosarcoma cells for the redox state of cell-surface protein thiols/disulfides. Overexpression of PDI resulted in 3.6-4. 2-fold enhanced secretion of PDI and 1.5-1.7-fold increase in surface-bound PDI. Antisense-mediated underexpression of PDI caused 38-53% decreased secretion and 10-33% decrease in surface-bound PDI. Using 5,5'-dithio-bis(2-nitrobenzoic acid) to measure surface protein thiols, a 41-50% increase in surface thiols was observed in PDI-overexpressing cells, whereas a 29-33% decrease was observed in underexpressing cells. Surface thiol content was strongly correlated with cellular (r = 0.998) and secreted (r = 0.969) PDI levels. The pattern of exofacial protein thiols was examined by labeling with the membrane-impermeable thiol reactive compound, 3-(N-maleimidylpropionyl)biocytin. Fourteen identifiable proteins on HT1080 cells were labeled with 3-(N-maleimidylpropionyl)biocytin. The intensity of labeling of 11 proteins was increased with overexpression of PDI, whereas the intensity of labeling of 3 of the 11 proteins was clearly decreased with underexpression of PDI. These findings indicated that secreted PDI was controlling the redox state of existing exofacial protein thiols or reactive disulfide bonds.

Angiostatin diminishes activation of the mitogen-activated protein kinases ERK-1 and ERK-2 in human dermal microvascular endothelial cells

Angiostatin is an endogenous inhibitor of angiogenesis that was isolated from tumor-bearing mice. It has been established that angiostatin inhibits endothelial cell proliferation; however, the underlying mechanisms remain to be elucidated. Here we report that angiostatin reduces transiently the phosphorylation of the mitogen-activated protein kinases ERK-1 and ERK-2 in human dermal microvascular cells, but not in human vascular smooth muscle cells or human dermal fibroblasts. We demonstrate that angiostatin diminishes ERK activation by basic fibroblast growth factor and vascular endothelial growth factor. Dephosphorylation of ERK and other tyrosine-phosphorylated proteins was blocked by pretreatment of the cells with sodium meta-vanadate, an inhibitor of protein tyrosine phosphatases, indicating that angiostatin signaling may require the activity of a tyrosine phosphatase. Concentrations of angiostatin that inhibited ERK activation also inhibited basic fibroblast growth factor-stimulated collagen gel invasion by endothelial cells, but did not affect endothelial cell proliferation. We thus show that angiostatin inhibits primarily the invasion of endothelial cells and exerts minimal (if any) effects on their proliferation. Invasion is a process that involves proteolysis, adhesion and migration, all of which have been linked to ERK signaling.

Matrix Metalloproteinases Generate Angiostatin: Effects on Neovascularization

Angiostatin, a cleavage product of plasminogen, has been shown to inhibit endothelial cell proliferation and metastatic tumor cell growth. Recently, the production of angiostatin has been correlated with tumor-associated macrophage production of elastolytic metalloproteinases in a murine model of Lewis lung cell carcinoma. In this report we demonstrate that purified murine and human matrix metalloproteinases generate biologically functional angiostatin from plasminogen. Macrophage elastase (MMP-12 or MME) proved to be the most efficient angiostatin-producing MMP. MME was followed by gelatinases and then the stomelysins in catalytic efficiency; interstitial collagenases had little capacity to generate angiostatin. Both recombinant angiostatin and angiostatin generated from recombinant MME-treated plasminogen inhibited human microvascular endothelial cell proliferation and differentiation in vitro. Finally, employing macrophages isolated from MME-deficient mice and their wild-type littermates, we demonstrate that MME is required for the generation of angiostatin that inhibits the proliferation of human microvascular endothelial cells.

Wound fluid from venous leg ulcers degrades plasminogen and reduces plasmin generation by keratinocytes

Plasminogen, the pro-enzyme of plasmin, aids various processes essential for normal, acute wound healing, such as fibrinolysis and cell migration. We have investigated if plasminogen is available to perform these functions in chronic wounds such as venous leg ulcers. We report that plasminogen is degraded by fluid from venous leg ulcers to a number of fragments, including kringle domains 1-3, an angiostatin-related protein. The enzyme responsible was inhibited by the serine protease inhibitor phenyl-methylsulfonyl fluoride, but was not inhibited by alpha1-anti-trypsin, an inhibitor of neutrophil elastase, by alpha2-anti-plasmin, an inhibitor of plasmin, or by the matrix metalloprotease inhibitor 1,10 phenanthroline. Plasminogen degraded by wound fluid was a weaker substrate than intact plasminogen for plasmin generation by the keratinocyte cell line HaCaT. These results suggest that serine protease activity in leg ulcer fluid degrades plasminogen and support the hypothesis that keratinocyte migration may be impaired in leg ulcers because of a reduced availability of intact plasminogen for plasmin generation.

Macrophage formation of angiostatin during inflammation. A byproduct of the activation of plasminogen

Angiostatin is a potent inhibitor of tumor angiogenesis and the growth of metastatic foci. Recent studies have indicated that neoplastic cells can generate angiostatin directly or in cooperation with tumor-associated macrophages. In studies reported here, we determined whether angiostatin is generated in mice under non-neoplastic settings. Utilizing murine RAW264.7 macrophages and thioglycollate-elicited peritoneal macrophages, we demonstrate that angiostatin-like fragments are generated as a byproduct of the proteolytic regulation of membrane-bound plasmin. Plasmin proteolysis and subsequent loss in membrane-bound plasmin activity requires active plasmin but was unaffected by inhibitors of metalloproteinases. Lysine binding fragments of plasmin, isolated from macrophage-conditioned media utilizing affinity chromatography, appeared as a major (48 kDa) and two minor bands (42 and 50 kDa) in SDS-polyacrylamide gel electrophoresis and were immunoreactive with anti-kringle 1-3 IgG. Each peptide begins with Lys77 and contains the entire sequence of angiostatin. The affinity isolated plasmin fragments inhibited bFGF-induced endothelial cell proliferation. Lavage fluid recovered from the peritoneal cavities of mice previously injected with thioglycollate contained angiostatin-like plasmin fragments similar to those generated in vitro. This is the first demonstration that angiostatin-like plasmin fragments are generated in a non-neoplastic inflammatory setting. Thus, in addition to regulating pericellular plasmin activity, proteolysis of plasmin generates inactive kringle-containing fragments expressing angiostatic properties.

Exposure of the cryptic Arg-Gly-Asp sequence in thrombospondin-1 by protein disulfide isomerase

Thrombospondin-1 is a matrix protein that inhibits proliferation, motility and sprouting of endothelial cells in vitro and angiogenesis in vivo. One mechanism by which thrombospondin-1 may influence endothelial cell biology is through interaction with the endothelial cell alphav beta3 integrin receptor. This interaction is mediated via a cryptic Arg-Gly-Asp sequence in the C-terminal Ca2+-binding region of thrombospondin-1. Exposure of the Arg-Gly-Asp sequence is controlled by disulfide interchange events in the Ca2+-binding loops and C-globular domain. Limited reduction of thrombospondin-1 by dithiothreitol exposes the Arg-Gly-Asp sequence which can bind to the alphav beta3 integrin receptor and support endothelial cell spreading (X. Sun, K. Skorstengaard, D.F. Mosher, J. Cell Biol. 118 (1992) 693-701). Our aim was to identify possible physiological reductants that can mediate Arg-Gly-Asp exposure. We now report that protein disulfide isomerase, which is known to catalyze disulfide interchange in thrombospondin-1 and change its enzyme inhibitory properties and its binding to monoclonal antibodies, was secreted by bovine aortic endothelial cells and deposited on the cell surface. There was an average of approximately 2.2 fg of protein disulfide isomerase on the surface of a bovine aortic endothelial cell. Treatment of thrombospondin-1 with purified protein disulfide isomerase enhanced adhesion of endothelial cells to thrombospondin-1 in an Arg-Gly-Asp-dependent manner through the alphav beta3 integrin receptor and supported cell spreading. Both Ca2+-depleted and Ca2+-replete thrombospondin-1 were substrates for protein disulfide isomerase. These results suggest that endothelial cell derived protein disulfide isomerase may regulate Arg-Gly-Asp-dependent binding of thrombospondin-1.

Expression of human macrophage metalloelastase gene in hepatocellular carcinoma: correlation with angiostatin generation and its clinical significance

Macrophage metalloelastase, a member of the human matrix metalloproteinase family, is believed to play an important role in angiostatin generation, which, in experimental studies, has an antiangiogenic function and is a key molecule in tumor dormancy. However, no clinical studies have been reported regarding the correlation between human macrophage metalloelastase (HME) gene expression and angiostatin production. Therefore, the present study was designed to evaluate the HME messenger RNA (mRNA) expression and angiostatin generation in hepatocellular carcinoma (HCC). Tumorous and contiguous nontumorous tissues were obtained from 40 HCC patients who underwent curative partial hepatectomy. By using Northern blot hybridization, HME mRNA was detected in 25 of the 40 HCC samples and, in all of these cases, the expression in tumorous tissues was stronger than in the nontumorous tissues. In situ hybridization identified the HCC cells as mainly responsible for the signals shown in Northern blot. Angiostatin was detected by Western blot mainly in tumors and showed significant association with HME mRNA expression in tumorous tissues (P = .0008). The patients whose tumors did not express HME mRNA and, thus, did not produce angiostatin, demonstrated poorer survival than those whose tumors showed high expression of HME mRNA and angiostatin generation (P = . 002). The univariate and multivariate analyses revealed that HME mRNA expression is a new and independent variable affecting overall survival (P = .001 and P = .03, respectively). These findings show that the HME gene is expressed in HCC being significantly associated with angiostatin generation by such tumors and that HME mRNA expression may serve as a new molecular prognostic marker in HCC patients after partial hepatectomy.